How do you pick an rocket engine?

[The Wet Dog]

What stats do you look for when selecting an engine for a purpose if that makes sense.

[MechBFP]

High sea level ISP is good for boosters during launch and sometimes for atmospheric landers. High vacuum ISP engines are much better for outer space and bodies with no atmosphere.

For launchers or landers, you also want a decent TWR of at least 1.2, so the thrust value matters as well.

[GunsForBucks]

For me a lot will depend on the mass I am moving. Then factoring in the thrust power in vacuum or in atmosphere.

But honestly the numbers they give are all relative. In the end you have to experiment and decide what works best for that build.

[Wobbly Av8r]

Originally posted by Minotaur1501:

What stats do you look for when selecting an engine for a purpose

All of them!

I know - not the answer you want to see, but poke around a little and you'll see that each engine has its strong and weak points. Half the fun is exploring them!

In general:

• Solid rockets should be used only as boosters (not upper stages or "saved" for space flight).
  
• When able, select the most efficient engines (highest ISP value) also dependent on whether the engine is used as a booster or upper stage or interstellar engine.
  
• How much 'bang for the buck' do you need, i.e how much weight are you lifting? Don't forget that sometimes clustering a few smaller engines can be better than one larger, more expensive engine.
  
• Do I need the engine to gimbal or generate electricity?

The Engine Parts Wiki[wiki.kerbalspaceprogram.com] is handy if you use the sorting buttons on the top of each category to list the engines relative to their different qualities in an ascending or descending order, such as TWR or maximum thrust, etc.

[Kyle]

There are a lot of factors involved. ISP is a measurement of the engine's fuel efficiency. A higher number uses less fuel. Most engines are going to have very different values for ISP in atmosphere VS space/vacuum.

You would try to use more efficient atmosphere ISP engines for early stages, then use a high vacuum ISP engine for later stages. You don't have to be all the way out of atmosphere when you switch to a Vaccum Isp engine, just to clarify. Air pressure drops exponentially, so there is almost no air in the outer layers (let's say 50 km or higher)

You want to keep an eye on your Thrust-Weight Ratio (TWR) of each stage. You find that by clicking on the orange number of that stage on the side of the screen in the VAB.

The short way to explain it is to keep your TWR above 1, or it won't be able to take off. Too much weight, not enough thrust. As mentioned above, 1.2 TWR or higher for your launch stages is desired, to make sure it works. Vacuum stages TWR will change after launch, and technically doesn't have to be too high unless you care about truly landing it on a planet/moon with no atmosphere.

Keep in mind this is all rocket science, it takes time to even figure out what you need to learn about/improve. But it's awesome.

[The Wet Dog]

I wish i could set more than one comment as an answer because you are all so helpful!

[Kyle]

This is one of the better forums on the internet for sure. The Wiki is also a great source of knowledge, I spend many hours there comparing and planning. It can be much faster to look at the lists there than in-game.

[The Wet Dog]

Originally posted by William:

This is one of the better forums on the internet for sure. The Wiki is also a great source of knowledge, I spend many hours there comparing and planning. It can be much faster to look at the lists there than in-game.

The wiki doesn't really help because I am using modded engines that I have not used before so I wanted to know how to pick an engine based on stats.

[CBR JGWRR]

For general guidance:

Mass: lower is better
Tolerance: higher is better for all three
Max. temp: higher is better
Thrust ASL: higher is better for lift-off and lower-stage engines
Thrust VAC: higher is better for anything in high atmosphere or vacuum
Cost: lower is better

Then if you right mouse click it, you'll get more information, first two are repeats.
Min. thrust: lower is better (gives you more control)
Engine Isp. : higher is much better (especially for interplanetary vessels)
Propellants: lower usage is better, but higher thrust engines will inevitably use more
Gimbal: higher is better (usually, sometimes you want to limit this)

Jet engines will also state stationary and maximum thrusts, while solid fuel engines will also state the quantity of fuel.


Edit: The Kerbal way is to just attach it and see if it works.

[Yuki]

High Gimbal Engines: You should not need them with a proper designed rocket- for launch vehicles. If you got trouble keeping your rocket aimed, use one in the center. Never use high gimbal engines for the sides/boosters.
Same goes for "shuttles" (the shuttle engines are among those with the highest gimbal): Dont use them spread too far, but more close to the center. Thats where they work properly, otherwise your craft will just become "wacky" if you give them too much leverage. To the point that SAS will do more harm than help.

TWR: You will probably want to have some more at the beginning. Opinions differ a lot here, but in another thread we already proved that a TWR of 2-3 in the beginning makes for the most efficient launches (<3k DV to LKO and back).
Once you go "ballistic" however you can even use LESS than 1.0. And many efficient space engines have a rather low thrust.
You will still be accelerating and hence getting altitude.
So dont be afraid to rely on a 0,5 TWR stage- in the upper athmosphere while aiming just short above the horizon. Accelerating forward will also increase your max altitude, your AP here, and every bit of thrust will accelerate. The exact point on when you are able to rely on <1.0 TWR is however...very dependent on your ascent profile, on your speed and angle. Just give it a try.

ASL vs VAC:
Different engines have different curves as to when they become efficient. Some sooner, some later. But one of the, if not THE "trademark" space-only engine, the NERVA, can be fired as low as 30km and already be called efficient. Good to know when constructing SSTO-Spaceplanes, which's other jet-engines often flame out at- you name it - about 30 km! Whiplash-Nerva is a really awesome SSTO combo. Who needs rapiers anyways :P

Other examples include the terrier, a "normal" LFO space-grade engine. Many of my satelite launches have only a terrier for the second stage, ~30km all the way up to orbit.

Price:
The price of an engine is less important IF- you recover it. Or re-use it. If you land on your airstrip you get all back. Less the more far away you land.
Dont be afraid to use those awesome pricey shuttle engines. On a shuttle that returns. But never ever use them on a disposeable launcher.
NERVA are expensive? Sure, but if you use them for a shuttle that traffics between Mun and a LKO station- well worth it.